The present invention relates to a method and system for low-cost, reliable semiliquid die casting of high performance mechanical components, particularly vehicle injection system parts, from rheocast light alloy ingots.
Italian Patent n. 1.119.287 filed on 20 Jun., 1979, and entitled: "Process and device for preparing a metal alloy mixture comprising a solid and liquid phase", the content of which is expressly incorporated herein by reference, relates to a static mixer for bringing a metal alloy into a "semiliquid" state in which the alloy, though already within the solidification range, can be cast, and presents a homogeneous composition and appearance as though still fully liquid.
More recent studies (R. L. Antona-R. Moschini: Met. Sci. Technol., 1986, vol.4 (2), p. 49-59; M. C. Flemings: Met. Transactions B, June 1991, vol.22 (B), p. 269-293) have shown that, when solidified, semiliquid cast light alloys--known as "rheocast" alloys--present a characteristic microstructure--a globular as opposed to the normal dendritic structure--resulting in a characteristic behaviour of the alloy when restored to a temperature within the solidification range. More specifically, rheocast alloys with a globular structure tend to segregate eutectic liquid and reassume a semiliquid state in which the alloy presents a characteristic "dessert cream" consistency.
In the "semiliquid" state, rheocast alloys have also been found to be pseudoplastic in the sense that viscosity varies (decreases) alongside a variation (increase) in the applied shear rate. According to Italian Patent Application n. TO91A000299 filed on Oct. 4, 1991 by the present Applicant and entitled: "Process for producing high mechanical performance die castings via injection of a semiliquid metal alloy", the content of which is expressly incorporated herein by reference, the pseudoplastic behaviour of rheocast alloys is exploited for producing good quality, sound die castings from semiliquid alloys.
Transferring semiliquid die casting technology to mass production, however, presents more than a few problems. Foremost of these is the difficulty in ensuring the die casting machine has a continuous supply of ingots within a suitable temperature range. Such a continuous supply within the suitable temperature range is necessary to prevent no-load injection and hence damage to the machine for lack of the ingot, and to prevent the alloy from being injected in less than optimum rheological conditions (due to over- or underheating), the latter being a fairly common occurrence due to the widely varying Reynolds number relative to the variation in the viscosity of the metal alloy for a given gate section of the die casting machine.
The method described by Flemings (M. C. Flemings: Met. Transactions B, June 1991, vol. 22 (B), p. 269-293), whereby rheocast ingots are produced by magnetic agitation and inductive preheating to the die casting temperature, imposes an extremely narrow preheat temperature range (i.e., a range corresponding to the presence of 50% solid fraction .+-.0.5.degree. C.), poses problems as regards handling of the ingots (induction heating rules out the use of containers, so that the ingots must be handled as solids), and poses serious difficulties in obtaining complete finished castings with the required degree of soundness. And even if this were possible, the castings would contain too many gaseous inclusions for them to be heat treated.